This invention relates to certain anaerobic adhesive and sealant compositions being capable of expanding during and as a result of post-curing. More specifically, this invention relates to adhesive compositions which are particularly useful in applications where polymerization shrinkage is problematic.
Adhesive and sealant compositions based on acrylate and methacrylate monomers polymerizable by free-radical initiation are known in the art. Likewise, anaerobic compositions are also well known in the art and are characterized by their ability to remain liquid in the presence of air, but cure to a strong adhesive bond when air is excluded, such as by assembling a mated nut and bolt to which the composition has been applied. One major drawback of both anaerobic and structural acrylic adhesives is their tendency to shrink substantially during polymerization. Such shrinkage is highly undesirable for many applications which depend on strict tolerances and which undergo or are exposed to significant stress/strain forces.
It is known that a number of factors contribute to shrinkage during polymerization. One such factor is that monomer molecules are generally located at a van der Waals' distance from one another, while in the corresponding polymerization state, the monomeric units move to within a covalent distance of one another. This change in distance brings the atoms much closer to one another in the polymer than they were in the original monomer. Other shrinkage factors can be attributed to the change in entropy in going from monomer to polymer, the free volume present in amorphous polymers, as well as the ability of the monomer and/or polymer to pack crystals if they are present in either phase. Additionally, in condensation polymerization, the shrinkage is partially related to the size of molecule that is eliminated. It is known, for example, that shrinkage with the synthesis of certain polymers can vary widely. For example, during the synthesis of nylon 6,6 the shrinkage can vary from 22% to 66% depending on the condensation reactants.
In polymerization reactions involving ring-opening, less shrinkage is generally encountered than with simple addition polymerization. This is believed to be due to the counter balancing effect of the ring which is opening and the corresponding atoms moving from a covalent distance to a near van der Waals distance. The result is that the monomer units which move from a van der Waals distance to a covalent distance during polymerization, are counter-balanced by the atoms in the opened ring moving from a covalent distance to a van der Waals distance. The result is that the bigger the ring, the closer to a true van der Waals distance is approached during ring opening and the smaller the shrinkage during overall polymerization. Attempts to polymerize monomers which had at least two rings available for opening for every new bond formed in the backbone have been made with the intention of minimizing shrinkage. For example, it has been shown that certain bicyclic materials will polymerize with either no change in volume or in some cases slight expansion, depending on the type and amount of monomer present.
However, with the exception of reports involving ring-opening polymerization reactions involving specific spiro ortho esters, shrinkage has continued to be a major problem in anaerobic and acrylic chemistry. There is a need, therefore, for adhesive compositions which will not only undergo polymerization with substantially no shrinkage, but also have the capability of controlled expansion subsequent to polymerization, thereby overcoming the disadvantages associated with the prior art compositions. The inventive compositions are able to fulfill this need and are capable of serving an enhanced role in a variety of adhesive applications where shrinkage is problematic or the need for expansion exists and the requirement for high bond strength under elevated temperatures is desirable.